This invention relates to a process for producing high-strength, high-electroconductivity copper-base alloys suitable for use as materials to make electric and electronic components such as leadframes.
With the recent development of the electronics industry, the use of materials to make electric and electronic components such as leadframes is growing and it is required that they have more reliable characteristics whereas they can be manufactured at a lower cost.
The term "leadframe" as used herein may be defined as "a single frame structure that supports the leads on IC chips during or after the fabrication process". Leadframes are required to have the following characteristics.
(1) Good Thermal and Electrical Conductivities:
One of the primary functions of leadframes is to prevent the deterioration of Si chips by diffusing the heat generated on the chips. To increase the efficiency of heat diffusion, the leadframes must have good thermal conductivity and, what is more, they are required to have good electrical conductivity in order to reduce heat generation in the lead portions. Since a linear relationship is generally recognized between thermal and electrical conductivities, the measurement of electrical conductivity will suffice for the evaluation of thermal conductivity.
(2) High Strength:
Leadframes, and outer leads in particular, are required to have sufficient strength to support the IC chips moldings including during and after the fabrication process. The criteria for evaluation include high tensile strength, high yield point and sufficient stiffness.
(3) Sufficient Heat Resistance:
Leadframes are expected to be subject to a certain degree of heat during or after the fabrication process and, therefore, they must be sufficiently heat resistant to ensure against deterioration in strength under thermal loads. In practice, however, unduly high heat-resisting temperatures are expected to cause economic disadvantages such as the need to employ high annealing temperatures during the preparation of raw materials. Therefore, leadframes will suffice for practical purposes if they do not soften upon heating at 350.degree. C. for a few minutes.
(4) Amenability to Bending Work:
In almost all cases, leadframes are subjected to bending in the lead portions and hence are required to have amenability to bending work. The methods for evaluation include V and W bending, as well as cyclic bend test.
(5) Good Platability and Solderability:
Leadframes are often provided with a Ag/Au plate on the inner leads and with a solder plates on the outer leads. Hence, they need satisfy good platability and solderability requirements, as well as the requirement that the applied plate and solder be resistant to weathering.
(6) Low Cost:
While leadframes are required to possess these characteristics, no single materials have heretofore been available that satisfy all of these requirements and that are yet inexpensive. In addition, the reliability requirement of semiconductors has recently become more rigorous than before and the use of surface-mounting type (SMT) devices which can be fabricated in smaller sizes has also increased. As a result, the ability to withstand the heat of solder without delamination has become a by far more important characteristic than it has been in the past. Leadframes are mounted on printed-wiring boards by soldering and in an operating environment the temperature of the soldered part will rise to a maximum of 120-150.degree. C. due both to the external temperature and to the heat generation during the application of an electric current. If the semiconductor device is exposed to such environment for a prolonged time, the solder layer will separate from the leadframe and the device is no longer operational. Hence, in the case where high device reliability is required and particularly with surface-mounting types such as QFP (quad flat L-leaded package) and PLCC (plastic leaded chip carrier) which are mounted by area contact rather than by insertion into through-holes in the printed-wiring board, the ability to withstand the heat of solder without delamination is an extremely important design factor.